It is well known that thermoplastic containers may be economically formed from a sheet of plastic material by the steps of (1) advancing a sheet of plastic material through an oven, thereby softening the material to a malleable state; (2) advancing the plastic sheet over a mold having the desired geometry of the container; (3) drawing the softened plastic sheet into the contours of the mold by means of a vacuum; (4) cooling the plastic sheet in the impression of the mold; (5) releasing the molded plastic containers from the mold; and (6) advancing the container to a trim station to trim individual containers from the continuous roll. These steps are used to manufacture virtually all modern thermoplastic containers. Alternative container designs may be accommodated by changing the shape of the mold.
Depending on the purpose of the container, it may be desirable to provide the container with an interlocking lid. Such containers generally interlock by means of male locking members on the container interlocking with a corresponding indented surface or aperture on the lid. The male locking members are generally formed by providing the mold with one or more male projections which are defined by an outward extension from the mold at its top edge and an undercut or inward extension back toward the mold a designated distance below the top edge of the mold. Thus, the length of the male locking members in a transverse direction is defined by the distance between the undercut and the furthest outward surface of the male projection, while the length of the male locking member in a longitudinal direction is defined by the distance between the undercut and the top edge of the male projection.
A significant disadvantage associated with the prior art method of forming male locking members is that the hardened plastic locking member becomes difficult to remove from the projection over which it was formed. This problem occurs because, upon removal of the container from the mold, the undercut portion of the locking member encounters and becomes blocked by the farthest outward surface of the male projection extending from the mold. Obviously, this problem becomes more exaggerated in proportion to the degree that the undercut extends inwardly from the farthest outward surface. The greater the degree of undercut, the more difficult it is to remove the plastic container from the mold. Thus, known prior art thermoplastic containers are limited to using locking members having a relatively small degree of undercut so that they may be easily removed from the mold. As a result of using locking members with a small degree of undercut, prior art containers cannot be relied upon to lock and stay locked during shipping and handling.
Accordingly, the present invention is directed to providing an apparatus and method for forming a locking member on a container with a larger undercut that overcomes or at least reduces the effects of one or more of the problems set forth above.